Oxidation of Aqueous Phosphorous Acid Electrolyte in Contact with Pt Studied by X-ray Photoemission Spectroscopy

The oxidation of the aqueous H3PO3 in contact with Pt was investigated for a fundamental understanding of the Pt/aqueous H3PO3 interaction with the goal of providing a comprehensive basis for the further optimization of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Ion-exchange chromatography (IEC) experiments suggested that in ambient conditions, Pt catalyzes H3PO3 oxidation to H3PO4 with H2O. X-ray photoelectron spectroscopy (XPS) on different substrates, including Au and Pt, previously treated in H3PO3 solutions was conducted to determine the catalytic abilities of selected metals toward H3PO3 oxidation. In situ ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) combined with the “dip-and-pull” method was performed to investigate the state of H3PO3 at the Pt|H3PO3 interface and in the bulk solution. It was shown that whereas H3PO3 remains stable in the bulk solution, the catalyzed oxidation of H3PO3 by H2O to H3PO4 accompanied by H2 generation occurs in contact with the Pt surface. This catalytic process likely involves H3PO3 adsorption at the Pt surface in a highly reactive pyramidal tautomeric configuration.


Topography and surface roughness determination of planar Au and planar Pt electrodes
To verify that the planar Au and planar Pt electrodes used in this study possess a low and comparable surface roughness with each other, atomic force microscopy (AFM) was performed for a detailed insight into these electrode's topography and surface roughness.In addition, electrochemically active surface area (ECSA) determination was made through copper (CuUPD) and hydrogen (HUPD) underpotential depositions, as it provides information about the whole electrode compared.The AFM and the ECSA  From the AFM scans, comparable surface roughness was observed for both electrodes: planar Au has an average surface roughness of 3.5 nm ± 1.2 nm, while planar Pt has an average surface roughness of 3.5 nm ± 0.6 nm.In the cyclic voltammetry curves of the CuUPD measurements on the planar Au and Pt thin film electrodes, the shaded regions correspond to the stripping/desorption of the adsorbed underpotentially deposited ions (i.e.Cu 2+ and H + ions for CuUPD and HUPD, respectively).The ECSA was estimated through the total charge (QUPD/total) determined by the current integration as depicted schematically in the shaded region divided by the scan rate, and subsequently normalized to the specific charge of the underpotentially deposited monolayer of ions (θ ref ) on Pt, and summarized in Table S1.
All measurements for the ECSA determination were conducted with a Pt mesh counter electrode (99.9%,Alfa Aesar) and a reversible hydrogen reference electrode (Mini HydroFlex, Gaskatel).Prior to the measurements, the electrolytes were deaerated by purging with N2 for 30 minutes.
Table S1.ECSA of planar Au and planar Pt electrodes used in the experiment.Ageo is the geometrical area of the electrode in the measurement, QUPD/total represents the total charge of underdeposited ions, θ ref is specific charge of the underpotentially deposited monolayer of ions on Pt.The roughness factor of the electrode was determined by normalizing the ECSA to the Ageo.

Electrode
Ageo (cm Interestingly, the roughness factor determined from electrochemical measurement shows that the planar Pt electrode has a higher roughness factor compared to the Au electrode, unlike the roughness factor determined from the AFM.From the hydrogen desorption of the UPD, ECSA was determined ~7.52 cm 2 .The roughness factor of the Pt black electrode was estimated from the ratio of ECSA to the geometrical area of the electrode used for the measurement (Ageo ~1.26 cm 2 ), in which the electrode displayed a roughness factor of ~5.97.

Fitting parameters for the XPS on the acid-treated electrodes
Table S3.Peak area and the calculated molar ratio of species from the XPS of the P 2p core level of the different substrates (planar Au and planar Pt) dipped into H3PO3 or H3PO4 solutions, as given in Figure 2 in the main text.The binding energy of the peak, FWHM, and Voigt model parameters (γ of solid reference are given in Table S2).electrodes, the open circuit potential (OCP) of these electrodes in 5 mol dm -3 H3PO3 solution was continuously monitored for ~15 hours.Prior to the experiments, the electrode was cleaned by immersion in 2 mol dm -3 H2SO4 for 6 hours, followed by thorough rinsing using Milli-Q water.Subsequently, the electrodes were activated in N2 purged 5 mol dm -3 H3PO3 electrolyte through potential cycling within the water stability window: +0.05 V and +1.0 V vs. RHE, with a scan rate of 50 mV s -1 , until no changes were observed in the region corresponding to the hydrogen desorption region (around +0.05 V to +0.4 V during positive-going potential sweep).Following the activation, the OCP was monitored in a 5-second intervals for approximately 15 hours until a steady state response was achieved.Throughout the OCP recording, the electrolyte was constantly stirred (rotation speed of 380 rpm, using IKA C-MAG HS7, magnetic stirrer) to ensure sufficient intermixing of the H3PO3 electrolyte in the proximity of the Pt electrode, where the surface-catalyzed oxidation might occur.The stirring reduces the time required for H3PO3 to reach the electrode from the bulk solution and enables a qualitative analysis within a shorter duration of OCP monitoring.This is because changes in the OCP are indicative of alterations in the electrode|electrolyte interface, which may result from reactions at the interface (such as the oxidation of H3PO3 to H3PO4 at the Pt surface).The OCP was recorded using a BioLogic SP-300 double channel potentiostat, with a Pt mesh (99.9%,Alfa Aesar) counter electrode, a reversible hydrogen reference  Please note that since the OCP corresponds to a mixed potential at which the sum of all anodic currents is equal to the sum of absolute values of all cathodic currents, the OCP reflects the rate of the individual surface processes taking place at the electrode surface.These rates are influenced by numerous interconnected conditions and phenomena, and at the initial stage of the OCP, the conditions are quite complex.Hence, in the present discussion, focus is given to the general trend of OCP which suggests blocking of the electrodes due to the oxidation of H3PO3 on the electrode surface, as previously discussed.
on the chemical oxidation of aqueous H3PO3 to H3PO4 to a certain extent, it does not allow a precise estimation of the reaction rate.The limitation arises because OCP provides the information from the state of the electrode|electrolyte interface, which is influenced by various factors such as the rate and type of the electrode reactions, as well as the mass transport of H3PO3 from the bulk solution to the interface.As the impact of these factors remains unknown, in this study OCP is solely used to gain qualitative insights into the chemical oxidation process and its subsequent effects on Pt electrodes.electrolytes were made at the pressure of 22 mbar, while measurements with 5 mol dm -3 solution were performed at the pressure of 18 mbar.
In the measurement positions, the signal arising from the electrode (i.e.Pt 4f core level from Pt black electrode) and electrolyte (i.e.liquid phase water in the O 1s core level, from the aqueous electrolyte) could be observed.This shows that the region probed during the AP-HAXPES measurements, indeed corresponds to the electrode|thin layer electrolyte interface.All measurements were conducted at the open circuit potential.Fitting parameters are given in Table S4 and Table S5 in the following.

Estimation of electrolyte layer thickness on the electrode surface for the in-situ AP-HAXPES coupled with the "Dip-and-pull" method
The estimation of the electrolyte layer thickness was made by deriving the Lambert-Beer equation, as presented in Ref. 5 , which takes into account: (i) the intensity of photoelectron flux generated from the electrode, which has been attenuated by the electrolyte layer, (ii) the intensity of photoelectron flux arising from the attenuating electrolyte, as well (iii) the inelastic mean free path (IMFP) of the photoelectrons, and (iv) the density of the electrode and electrolyte.This gives the following equation: Eq. S1.
2  is the thickness of the electrolyte layer (in nm).  is the inelastic mean free path (IMFP) of the photoelectron for substance x (in nm).The value for   2  was adapted from the calculation by Emfietzoglou and Nikjoo 6 (~8.34 nm), while the value for   was determined by the TPP2M equation 7 (29.48nm).  corresponds to the peak area of component x.The   2  was integrated from peak area of liquid phase H2O in the recorded O 1s spectrum, while   was integrated from peak area of Pt 4f7/2 core level under the same conditions.  is the number of atoms/molecules per unit volume for the substance x.For   2  and   , 33.4 nm -3 and were 66.19 nm -3 used, respectively, as used by Ref. 8 .The estimated thicknesses of the thin electrolyte layers found at the probed Pt|aqueous electrolyte interfaces across different electrolyte solutions have been detailed in Table S6.
Table S6.Electrolyte layer thickness (telectrolyte = tH2O) on the electrode surface in the in situ AP-HAXPES combined with the "Dip-and-pull" method It is important to note, that the estimation method employed here is typically used for planar electrodes and may not be as precise for electrodes with higher surface roughness, such as Pt black.Consequently, the estimated electrolyte layer thicknesses for rougher electrodes carry larger uncertainties when compared to planar electrodes.Nevertheless, despite these uncertainties, this estimation is conducted to provide an approximate value of the electrolyte layer thickness, as the oxidation of aqueous H3PO3 on Pt is expected to be correlated with the thickness of the electrolyte layer.It is worth noting that despite the potential limitations for rougher electrodes, the estimated thickness of the electrolyte layer in this study aligns well with the values reported in previous studies [8][9][10][10][11][12] Electrolyte telectrolyte (nm)

Validation of continuous thin film electrolyte from the probed Pt|thin electrolyte interface to the bulk electrolyte
To verify that the probed spot at the electrode|electrolyte was connected to the bulk electrolyte solution, O 1s and Pt 4f spectra of the electrode|thin layer electrolyte were recorded during potential application of +0.05 V and +1.0 V vs. RHE to the working electrode.Subsequently, the shift of the spectra binding energy (B.E.) during these potential applications was compared to the expected ones.

Figure S8.
In situ AP-HAXPES coupled with the "Dip-and-pull" method was conducted on the Pt black|electrolyte interface during potential application of +0.05 V and +1.0 V vs. RHE, measuring: (A) the O 1s core level and (B) the Pt 4f core level.All measurements were recorded with an excitation energy of 3 keV.Measurements with 1 mol dm -3 electrolytes were made at a pressure of 22 mbar, while measurements with the 5 mol dm -3 solution were performed at a pressure of 18 mbar.
As shown in Figure S8, there is a spectral shift observed in the O 1s core level, while no shift is observed in the Pt 4f spectra.Due to the potential applied to the Pt electrode, a binding energy shift of the O 1s spectra arising from the electrolyte corresponding to the applied potential was observed.If the probed spot was disconnected from the bulk solution electrolyte, or if the electrolyte layer consist of many droplets, such a shift would be absent 9 .It is important to note, that since the electrode possesses common ground with the electron analyzer, the Pt 4f core level energy does not shift like the O 1s spectra 9 .For the measurement with 5 mol dm -3 H3PO4 solution, there is a drop in peak intensity corresponding to the liquid phase water (~532.0eV) between the application of +0.5 V and +1.0 V.This could indicate that the electrolyte layer was slightly unstable and thinned out during the measurement, although a good connection to the bulk electrolyte solution was still maintained.For all the other compounds the intensity of the peak assigned to the liquid phase H2O remains comparable for the +0.5 V and +1.0 V measurements (i.e. the thin electrolyte layer remains stable).S22

PtOx
Since the possibility of an oxidized Pt monolayer (i.e.PtOx) at the investigated electrode during the in situ AP-HAXPES measurement cannot be excluded, an estimation of the H3PO4 molar fraction resulting from the oxidation of H3PO3 by a such hypothetical PtOx monolayer was made.By comparing the theoretical molar fraction of H3PO4 resulting from the oxidation of H3PO3 by a monolayer of PtOx with the observed H3PO4 molar fraction observed in the AP-HAXPES measurement (given in Table S7), further insight into the oxidation process occurring during the in situ AP-HAXPES could be made.
For this estimation, firstly the number of H3PO3 molecules inside the electrolyte volume probed during AP-HAXPES was estimated.The estimation was performed by using Eq.S2.
3  3 represents the probed H3PO3 electrolyte volume, estimated from electrolyte thickness, telectrolyte (given in Table S6), and the probed area given by the beam spot, Abeam spot: approximately 400 µm × 700 µm.Subsequently, the number of PtOx forming a monolayer on the electrode surface probed in the AP-HAXPES was also estimated by Eq.S3.
= (       ) × (     ) Eq. S3 θPtOx corresponds to the specific surface charge of a PtO monolayer (420 µC cm -2 , according to ref. 13 ), Abeam spot is the probed surface area given by the beam spot (approximately 400 µm × 700 µm), F represents the Faraday constant (F = 9.648 × 10 4 C mol -1 ), and rfWE corresponds to the roughness factor of the working electrode (rfWE~ 5.97, see detail in section 2, Figure S2).Through Eq.S3, the number of PtO covering the whole surface area of the beam spot was estimated to be ~4.38 × 10 13 .

S23
Finally, an estimation of the H3PO4 molar fraction is made through the number of PtO in probed monolayer surface, to the number of H3PO3 molecules in the probed electrolyte volume (assuming each PtO is oxidizing one H3PO3 molecule).This estimated value is given in Table S8.As shown in Table S8, while a monolayer of hypothetical PtOx may fully oxidize 1 mol dm -3 H3PO3, the H3PO4 molar fraction in the 5 mol dm -3 H3PO3 due to the oxidation of H3PO3 by a PtOx monolayer is smaller compared to the H3PO4 molar fraction observed by the AP-HAXPES (see fractions in Table S3).This indicates that the oxidation cannot be explained solely by the presence of PtOx, and other processes must be responsible for the oxidation of the H3PO3 (e.g.oxidation of H3PO3 through H2O as proposed in the main text).Furthermore, it is important to note that, this assumption is made for an extreme situation, in which the generated H3PO4 stays on the electrode surface and it does not diffuses away from the proven volume.However, given the experimental duration in the probed spot (~1.5 hrs), diffusion likely occurs, which means that less H3PO4 produced from this phenomenon should be observed.Moreover, in the event that oxidation by PtOx occurs in a very short timescale, during the "dipping" of the electrode in the electrolyte (i.e.before "pulling" the electrode up to form the electrode|thin electrolyte layer for the in situ AP HAXPES), the H3PO3 might already by oxidized in this process.Hence, at the time of measurement, the H3PO4 formed by this process might already diffuse away.As result, even less influence of oxidation by PtOx should be observed in this case.

In situ "Dip-and-pull" AP-HAXPES measurements of the Pt-electrode|aqueous H3PO3 electrolyte interface
To verify the reproducibility of the obtained results, two data sets of in situ "Dip-and-pull" AP-HAXPES measurements of the Pt black|aqueous H3PO3 interface are compared.These measurements were performed under OCP conditions using both, 1 mol dm -3 H3PO3 and 5 mol dm   Furthermore, when comparing measurements performed at the Pt|aq.H3PO3 electrolyte interface (i.e., measurement (1) and ( 2)) to the experiment performed on a slightly thicker part of the electrolyte layer (i.e., measurement (3)), only a slight difference is observed between them.In both cases pronounced oxidation of aqueous H3PO3 can be observed.Note that for measurement (3), the electrolyte layer thickness could not be properly estimated via Eq.S1, since the signal corresponding to the Pt 4f core level of the electrode could not be measured.Nevertheless, considering that all of the measurement positions are located more than 0.5 cm above the surface of the bulk electrolyte in the beaker, it is likely that this spot possesses an electrolyte thickness of less than 50 nm, as previously suggested in Ref. 8 .

S26
Consequently, an increase in the local concentration of H3PO4 likely occurs at this measurement position (i.e. in the thin electrolyte layer of the formed meniscus), similar to what is observed in the other measurement positions, as discussed in the main text.
determinations of planar Au (left) and planar Pt (right) are shown in Figure S1.

Figure S1 .
Figure S1.AFM images of (A) planar Au and (B) planar Pt electrodes.(C) CuUPD on planar Au electrode and (D) HUPD of planar Pt electrode for ECSA estimation.
Figure S2.(A) Illustration and (B) scanning electron microscopy (SEM) image of the 15 nm thick Pt layer sputtered on top of Si-wafer, as a substrate for the deposition of Pt black working electrode.(C) Schematic and (D) SEM image of the Pt black deposited on top of the sputtered Pt shown in Fig. S2.A. (E).Potential profile and current densities drawn to the working electrodes (i.e.planar 15 nm sputtered Pt on Si-wafer, as shown in Fig. S2.A) during Chronopotentiometry (CP), for the electrodeposition of Pt black (Fig. S2.C).The experimental details for the preparation of Pt black preparation are given in the experimental section of the main text.(F) HUPD of Pt black electrode for the determination of ECSA and roughness factor of the Pt black electrode.CV was recorded with N2 saturated 0.1 M H2SO4, a Pt mesh counter electrode (99.9%,Alfa Aesar), and a reversible hydrogen reference electrode (Mini HydroFlex, Gaskatel) , with a scan rate of 50 mV s -1 .

S6 3 .S7 4 .
Figure S3.X-ray photoelectron spectroscopy (XPS) on P 2p core level of Au and Pt electrode previously treated in 5 mol dm −3 H3PO4, as given in Figure 2 in the main text, along with the curve fitting.The dashed lines correspond to the binding energies of P 2p3/2 of the solid H3PO3 and solid H3PO4 references.The peak contribution almost exclusively emerges from H3PO4, similar to the solid H3PO4 crystalline reference, indicating that the solution is stable on the electrodes.XPS on the acid-treated electrode was performed with Mg K excitation (1253.56eV) at a pressure < 5×10 -8 mbar.
electrode (Mini HydroFlex, Gaskatel), and 40 ml of electrolyte volume.The working electrodes possess a geometrical area of ~0.875 cm 2 .The recorded OCP data of 5 mol dm -3 H3PO3 on the planar Pt, planar Au, and Pt black electrodes are presented in Figure S4.

Figure S4 .
Figure S4.OCP monitoring of 5 mol dm -3 H3PO3 on Pt electrodes of different surface roughness (Top Panel): planar Pt and rougher Pt black; and on different planar metal electrodes (Bottom panel): planar Au and planar Pt.Experiments were performed with a constant stirring of 380 rpm.The displayed OCP values represent averages over a 50 seconds-interval (10 data points).

Figure S5 .
Figure S5.Cyclic voltammograms (CV) of (A) a planar Pt and (B) planar Au working electrodes in (top): 5 mol dm -3 (5 M) H3PO4 and (bottom): in 5 mol dm -3 (5 M) H3PO3.All CVs were recorded with the starting potential of +0.05 V vs. RHE, using the scan rate of 50 mV s -1 .The current response during the positive-going potential sweep in panel (A, bottom) is shown with a green arrow, while the violet arrow illustrates the current response during the negative-going potential sweep.The inset graph in panel (B, bottom) shows a magnified look at the lower potential region with the lower current response.Each CV was recorded with three different upper reversal potentials (+1.2 V, +1.4 V, and +1.6 V vs. RHE).Between each CV, several scans were taken until steady-state voltammograms were obtained and then recorded.The small gray arrow on (A, top) indicates H3PO3 oxidation peak (i.e.indicating minor contamination of the solution by H3PO3).Gray dashed lines indicate the peak potential of H3PO3 oxidation on the surface oxides for each planar electrode, as shown in the bottom figure of Figure S4.A and Figure.S4.B.

Figure S6 .Figure 6 .Figure 5 . 9 .
Figure S6.CV of 10 mmol dm -3 H3PO3 + 0.5 mol dm -3 H3PO4 on Pt black electrode, recorded for 11,000 cycles with a scan rate of 50 mV s -1 .Individual CVs are shown for every 500 cycles, along with the CV of 0.5 mol dm -3 H3PO4 on the same electrode, for comparison.In (A) the recorded current is normalized to the geometrical area of the working electrode, while in (B) the recorded current is normalized to the ECSA of the working electrode.

11 .
Fitting parameters and quantification for the P 2p core level in the in situ AP-HAXPES coupled with "Dip-and-pull" method Figure S9 illustrates the model used for estimating the number of H3PO3 molecules in the probed electrolyte volume.c in the Eq.S2 corresponds to the concentration of the solution (in Molar, i.e. mol dm -3 ), and NA is Avogadro's constant (NA= 6.022 × 10 23 mol -1 ).

Figure S9 .
Figure S9.Illustration of the model for estimation of H3PO4:(H3PO3+H3PO4) molar ratio resulting from oxidation from a monolayer of PtOx.
-3 H3PO3 electrolytes.The same experimental setup detailed in the experimental section of the main text is used for both experiments.Furthermore, apart from the measurements made at the Pt-electrode|aq.H3PO3 interface, additional "Dip-and-pull" in situ AP HAXPES were also performed in a slightly thicker part of the electrolyte layer.In this region, the AP-HAXPES signal arising from the electrode (i.e., the Pt 4f core level) was no longer observable.This measurement aimed to provide insights into the influence of electrolyte layer thickness on the state of the H3PO3 electrolyte.Results from these experiments are shown in Figure S10 and Figure S11.

Figure S10 .
Figure S10.In situ "Dip-and-pull" AP-HAXPES data of 1 mol dm -3 (1 M) H3PO3 on Pt black: (A) P 2p, (B) O 1s, and (C) Pt 4f core levels."G.P. H2O" and "L.P. H2O" in panel (B) represent: "gas phase H2O" and "liquid phase H2O", respectively.All measurements were performed at open circuit potential (OCP), at the pressure of 22 mbar, and incoming photon energy of 3 keV.Measurements (1) and (2)were performed at the Pt-electrode|aqueous H3PO3 interface, while measurement (3) was carried out at a slightly thicker electrolyte layer, where the signal arising from the electrode (i.e., the Pt 4f) could not be observed.Note that measurement (1) is the same measurement of the Pt black|1 mol dm -3 H3PO3 interface, discussed in the main text, and it is re-plotted for easier comparison with the additional measurements.

Figure S11 .
Figure S11.In situ "Dip-and-pull" AP-HAXPES of 5 mol dm -3 (5 M) H3PO3 on Pt black: (A) P 2p, (B) O 1s, and (C) Pt 4f core levels."G.P. H2O" and "L.P. H2O" in panel (B) represent: "gas phase H2O" and "liquid phase H2O", respectively.All measurements were performed at open circuit potential (OCP), at the pressure of 18 mbar, and incoming photon energy of 3 keV.Measurements (1) and (2) were performed at the Pt-electrode|aqueous H3PO3 interface, while measurement (3) was carried out at a slightly thicker electrolyte layer, where the signal arising from the electrode (i.e., the Pt 4f) could not be observed.Note that measurement (1) is the same measurement of the Pt black|1 mol dm -3 H3PO3 interface, discussed in the main text, and it is re-plotted for easier comparison with the additional measurements.

Figure S12 .
Figure S12.(A) Illustration of the working electrode used for the experiment: The top part of the working electrode is a Pt black electrode which is prepared on top of Pt sputtered Si substrate.The bottom part of the electrode is an un-sputtered Si substrate.(B) Two measurement configurations used in the experiment: "fully-immersed" working electrode configuration and "thin-film-only" configuration.(C) CV was recorded in 5 mol dm -3 H3PO3 with both measurement configurations.(D), (E) galvanostatic electrochemical impedance spectroscopy for determination of the cell resistance and IR compensation for the "fully immersed", and the "thin-film-only" configuration, respectively.

Table S2 .
Fitting parameters for the ambient-pressure hard photoelectron spectroscopy (AP-HAXPES) of the P 2p core level of the solid crystalline H3PO3 and H3PO4 references, as given in Figure2, in the main text.The P 2p doublet separation was kept at 0.84 eV for solid H3PO3 and 0.97 eV for solid H3PO4, *The Voigt model parameters (γ used for the fitting process are (0.37, 0.37) and (0.35, 0.35), for solid H3PO3 and solid H3PO4 references, respectively.S8

. Open circuit potential (OCP) monitoring of 5 mol dm -3 H3PO3 in contact with planar Pt, planar Au, and Pt black electrodes To
gain further insights into the oxidation of aqueous H3PO3 on the planar Pt, planar Au, and Pt black

Table S4 .
Fitting parameters for the AP-HAXPES of the O 1s core level of the Pt black|electrolyte interface shown in Figure S5.Peaks were fitted with a Voigt profile and a Shirley background.FWHM* and Voigt model parameters () of each species (gas phase H2O and liquid phase H2O) were kept constant during the fitting procedure.
*Full-width half maxima (FWHM) for the fitting process were kept constant at: 0.97 eV and 2.31 eV, for gas phase H2O and liquid phase H2O, respectively.Voigt model parameters () for the fitting are: (0.26, 0.26) and (0.64, 0.64) for the gas phase H2O and liquid phase H2O, respectively

Table S5 .
Fitting parameters for the AP-HAXPES on the Pt 4f core level in the Pt black|electrolyte interface given in FigureS5.The asymmetric Pt 4f peaks were fitted using a Doniach-Šunjić profile and Shirley background, for a meaningful fit with metallic core level.The area of Pt 4f5/2 component was kept to 75 % of the area of Pt 4f7/2 component 4 .Moreover, with the assumption that the same species should arise under all the different conditions (i.e.Pt 0 ), the position of the peak, FWHM, and the Doniach-Šunjić model parameters ( and) were kept constant for the fitting.

Table S7 .
Fitted P 2p core level areas for the AP-HAXPES the Pt black|electrolyte interface as given in Figure 4.A and 4.B in the main text.The B.E. of the peak maxima, FWHM, and Voigt model parameters (γ were kept similar to the solid reference (as given in TableS2).

Table S8 .
Estimation of H3PO4 molar fraction resulting from the oxidation of H3PO3 by a monolayer PtOx.